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MySQL执行计划中的各个参数及含义指的是什么

发布时间:2022-01-13 14:55:02 所属栏目:MySql教程 来源:互联网
导读:这期内容当中小编将会给大家带来有关MySQL执行计划中的各个参数及含义指的是什么,文章内容丰富且以专业的角度为大家分析和叙述,阅读完这篇文章希望大家可以有所收获。 查看执行计划 可以使用explain + SQL或者desc + SQL 1、使用explain查看执行计划 mys
       这期内容当中小编将会给大家带来有关MySQL执行计划中的各个参数及含义指的是什么,文章内容丰富且以专业的角度为大家分析和叙述,阅读完这篇文章希望大家可以有所收获。
 
查看执行计划
       可以使用explain + SQL或者desc + SQL
 
       1、使用explain查看执行计划
mysql> explain select * from employee;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | employee | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | NULL  |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
2、使用desc查看执行计划
mysql> desc select * from employee;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | employee | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | NULL  |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
一、ID
select查询的序列号;包含一组数字,表示查询中执行select子句或操作表的顺序。
 
id的值分为三种情况:
 
id完全相同;
 
id完全不同;
 
id部分相同部分不相同。
 
1、id完全相同
对三个表进行关联
 
EXPLAIN
SELECT *
FROM   EMPLOYEE   E
      ,DEPARTMENT D
      ,CUSTOMER   C
WHERE  E.DEP_ID = D.ID
       AND E.CUS_ID = C.ID;
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref           | rows | filtered | Extra                                              |
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+----------------------------------------------------+
|  **1** | SIMPLE      | C     | NULL       | ALL    | PRIMARY       | NULL    | NULL    | NULL          |    1 |   100.00 | NULL                                               |
|  1 | SIMPLE      | E     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL          |    8 |    12.50 | Using where; Using join buffer (Block Nested Loop) |
|  1 | SIMPLE      | D     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | demo.E.dep_id |    1 |   100.00 | NULL                                               |
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------+------+----------+----------------------------------------------------+
3 rows in set, 1 warning (0.01 sec)
可以看到id这一列的数据都是相同的,都是1,执行顺序由上到下(先扫描c,在扫描e,最后扫描d)。
 
2、id完全不同
使用嵌套子查询
 
EXPLAIN
SELECT *
FROM   DEPARTMENT
WHERE  ID = (SELECT ID
             FROM   EMPLOYEE
             WHERE  ID = (SELECT ID FROM CUSTOMER WHERE ID = 1));
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | DEPARTMENT | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | EMPLOYEE   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
|  3 | SUBQUERY    | CUSTOMER   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.01 sec)
id为1、2、3,如果是子查询,id的序号会递增,id值越大优先级越高,优先被执行。
 
3、id部分相同部分不相同
使用子查询作为表
 
EXPLAIN
SELECT *
FROM   DEPARTMENT D
      ,(SELECT DEP_ID FROM EMPLOYEE GROUP BY DEP_ID) E
WHERE  D.ID = E.DEP_ID;
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------+------+----------+-----------------+
| id | select_type | table      | partitions | type | possible_keys | key         | key_len | ref       | rows | filtered | Extra           |
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------+------+----------+-----------------+
|  1 | PRIMARY     | D          | NULL       | ALL  | PRIMARY       | NULL        | NULL    | NULL      |    5 |   100.00 | NULL            |
|  1 | PRIMARY     | <derived2> | NULL       | ref  | <auto_key0>   | <auto_key0> | 5       | demo.D.id |    2 |   100.00 | Using index     |
|  2 | DERIVED     | EMPLOYEE   | NULL       | ALL  | NULL          | NULL        | NULL    | NULL      |    8 |   100.00 | Using temporary |
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
id部分相同,部分不相同。可以认为是一组,(id=1为一组,id=2为另一组)从上往下顺序执行;在所有组中,id值越大,优先级越高,越先执行;先执行完2组,然后执行1组,1组内,相同的依然是从上到下的执行顺序。
 
最终的执行顺序:
 
①id = 2先执行
 
②id = 1的组内的第一行先执行(d)
 
③id = 1的组内的第二行最后执行(<derived2>)
 
对于ID字段来说,相同的值则遵循从上往下的执行顺序;不同值时遵循大的先执行。
 
 二、select_type
查询类型,主要用于区别普通查询,联合查询,子查询等复杂查询
 
SIMPLE、 PRIMARY、SUBQUERY、DERIVED、UNION、UNION RESULT(DEPENDENT SUBQUERY、DEPENDENT UNION)
 
1、SIMPLE简单查询
简单select查询,查询中不包含子查询或者UNION。
 
对employee进行查询
 
EXPLAIN
SELECT * FROM EMPLOYEE;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | EMPLOYEE | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | NULL  |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
类型为simple,只对一个单表进行查询。
 
2、PRIMARY外层查询
查询中若包含任何复杂的子查询,最外层查询则被标记为primary。
 
EXPLAIN
SELECT *
FROM   DEPARTMENT
WHERE  ID = (SELECT ID
             FROM   EMPLOYEE
             WHERE  ID = (SELECT ID FROM CUSTOMER WHERE ID = 1));
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | DEPARTMENT | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | EMPLOYEE   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
|  3 | SUBQUERY    | CUSTOMER   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
Id=1的select_type为primary,对表department进行扫描,并且该查询是最外层查询,所以被标记为primary。
 
 3、SUBQUERY子查询
3.1、SUBQUERY不依赖于外部查询
在select或where中包含了子查询,子查询中的第一个select查询,不依赖于外部查询结果。
 
EXPLAIN
SELECT *
FROM   DEPARTMENT
WHERE  ID = (SELECT ID
             FROM   EMPLOYEE
             WHERE  ID = (SELECT ID FROM CUSTOMER WHERE ID = 1));
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | DEPARTMENT | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | EMPLOYEE   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
|  3 | SUBQUERY    | CUSTOMER   | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
Id = 2 和id = 3的类型为subquery,是一个子查询,被标记为subquery,并且由于该子查询不依赖与外部的查询结果,所以只显示subquery。
 
3.2、SUBQUERY依赖于外部查询
子查询的内容依赖与外部查询的内容,最明显的标量子查询。
 
EXPLAIN
SELECT ID
      ,(SELECT DEP_ID FROM EMPLOYEE E WHERE E.ID = D.ID) DEP_ID
FROM   DEPARTMENT D;
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type        | table | partitions | type   | possible_keys | key     | key_len | ref       | rows | filtered | Extra       |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
|  1 | PRIMARY            | D     | NULL       | index  | NULL          | PRIMARY | 4       | NULL      |    5 |   100.00 | Using index |
|  2 | DEPENDENT SUBQUERY | E     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | demo.D.id |    1 |   100.00 | NULL        |
+----+--------------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)
select中的子查询依赖与外部查询d表的结果,所以id = 2是子查询,并且是依赖于外部查询的子查询,被标记为DEPENDENT SUBQUERY。
 
 4、DERIVED衍生表(临时表)
在from列表中包含的子查询被标记为derived(衍生),把结果放在临时表当中。
 
EXPLAIN
SELECT *
FROM   DEPARTMENT D
      ,(SELECT DEP_ID FROM EMPLOYEE GROUP BY DEP_ID) E
WHERE  D.ID = E.DEP_ID;
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------+------+----------+-----------------+
| id | select_type | table      | partitions | type | possible_keys | key         | key_len | ref       | rows | filtered | Extra           |
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------+------+----------+-----------------+
|  1 | PRIMARY     | D          | NULL       | ALL  | PRIMARY       | NULL        | NULL    | NULL      |    5 |   100.00 | NULL            |
|  1 | PRIMARY     | <derived2> | NULL       | ref  | <auto_key0>   | <auto_key0> | 5       | demo.D.id |    2 |   100.00 | Using index     |
|  2 | DERIVED     | EMPLOYEE   | NULL       | ALL  | NULL          | NULL        | NULL    | NULL      |    8 |   100.00 | Using temporary |
+----+-------------+------------+------------+------+---------------+-------------+---------+-----------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
从id=2可以看到,先执行2,从table可以发现,被扫描的表是employee,是子查询的表,该子查询的结果被放在一个临时表里,被标记为DERIVED。
 
 5、UNION
 5.1、UNION不依赖于外部查询
若第二个select出现在union之后,则被标记为union,并且是不依赖于外部查询。
 
EXPLAIN
SELECT ID FROM EMPLOYEE UNION SELECT ID FROM DEPARTMENT;
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | EMPLOYEE   | NULL       | index | NULL          | PRIMARY | 4       | NULL |    8 |   100.00 | Using index     |
|  2 | UNION        | DEPARTMENT | NULL       | index | NULL          | PRIMARY | 4       | NULL |    5 |   100.00 | Using index     |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL   | NULL          | NULL    | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
该查询是由两个单独的select进行的union,不存在子查询,所以直接把id=2的类型标记为UNION,并且第一个的select会被默认的标记为primary,之后用来union的select都会在该select之前执行。
 
5.2、UNION依赖于外部查询
若第二个select出现在union之后,则被标记为union,并且是依赖于外部查询。
 
EXPLAIN
SELECT ID
      ,NAME
FROM   CUSTOMER
WHERE  ID IN (SELECT ID FROM EMPLOYEE UNION SELECT ID FROM DEPARTMENT);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type        | table      | partitions | type   | possible_keys | key     | key_len | ref  | rows | filtered | Extra           |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
|  1 | PRIMARY            | CUSTOMER   | NULL       | ALL    | NULL          | NULL    | NULL    | NULL |    1 |   100.00 | Using where     |
|  2 | DEPENDENT SUBQUERY | EMPLOYEE   | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index     |
|  3 | DEPENDENT UNION    | DEPARTMENT | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index     |
| NULL | UNION RESULT       | <union2,3> | NULL       | ALL    | NULL          | NULL    | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.01 sec)
还是和上面一样,emp表和dep表进行了union操作,不过现在放在了子查询里,现在的结果集和外面的查询的结果集有了联系。先执行的id=3为department表,并且该查询在union后面, 由于与外部查询有联系,所以被标记为DEPENDENT UNION;第二步执行的是id= 2的表employee,该部分是一个子查询部分,并且和外部查询有联系,所以是一个依赖子查询,标记为DEPENDENT SUBQUERY;最后执行的是id=1部分,最外层的查询,被标记为primary。
 
5.3、UNION在from子句
若union包含在from子句的子查询中,外层select将被标记为deriver(deriver表必须有别名),如果没有别名将会报错。
 
不写别名,会报错:
 
EXPLAIN
SELECT *
FROM   (SELECT ID
              ,NAME
        FROM   EMPLOYEE
        UNION
        SELECT ID
              ,DEPTNAME
        FROM   DEPARTMENT);
ERROR 1248 (42000): Every derived table must have its own alias
正确写法,写上别名:
 
EXPLAIN
SELECT *
FROM   (SELECT ID
              ,NAME
        FROM   EMPLOYEE
        UNION
        SELECT ID
              ,DEPTNAME
        FROM   DEPARTMENT) T;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | <derived2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   13 |   100.00 | NULL            |
|  2 | DERIVED      | EMPLOYEE   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | NULL            |
|  3 | UNION        | DEPARTMENT | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    5 |   100.00 | NULL            |
| NULL | UNION RESULT | <union2,3> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.00 sec)
和上面略有不同,现在是把union的子查询直接放在了from里面当作一个表,而不是根据一个表的字段去in或者exists连接,此时执行计划将会有所不同。
 
第一点:select * from (select * from b)的写法,必须给括号里的内容起一个别名,如果没有别名,将会报错“1248 - Every derived table must have its own alias”。
 
第二点:在使用这种写法时,会把括号里的查询生成一个衍生表(临时表),标记为DERIVED,进一步分析子查询,首先执行的是id=3的department表,因为在union 后面,所以对应该表的操作的select_type被标记为union;其次执行的是id=2的employee表,此处被标记为DERIVED;最后执行id=3,并且最外层是select * from (),被标记为primary,外部查询,而且由于是查询括号里的衍生表的记录,所以扫描的表是该衍生表deriverd2。
 
 6、UNION RESULT
从union表获取结果select;两个UNION合并的结果集在最后。
 
若第二个select出现在union之后,则被标记为union,并且是不依赖于外部查询。
 
EXPLAIN
SELECT ID FROM EMPLOYEE UNION SELECT ID FROM DEPARTMENT;
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | EMPLOYEE   | NULL       | index | NULL          | PRIMARY | 4       | NULL |    8 |   100.00 | Using index     |
|  2 | UNION        | DEPARTMENT | NULL       | index | NULL          | PRIMARY | 4       | NULL |    5 |   100.00 | Using index     |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL   | NULL          | NULL    | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
该查询是由两个单独的select进行的union,不存在子查询,先执行的id=2,对department进行扫描,在对employee表进行扫描,然后把这两个单独查询结果集进行合并,使用UNION RESULT表示union操作的合并的步骤,并且union是需要对两部分的数据进行排序然后在进行合并,所以只有当存在排序之后的合并才会有UNION RESULT。
 
 7、UNION ALL
7.1、UNION ALL不依赖于外部查询
若第二个select出现在union之后,则被标记为union,并且是不依赖于外部查询。
 
EXPLAIN
SELECT ID FROM EMPLOYEE UNION ALL SELECT ID FROM DEPARTMENT;
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | EMPLOYEE   | NULL       | index | NULL          | PRIMARY | 4       | NULL |    8 |   100.00 | Using index |
|  2 | UNION       | DEPARTMENT | NULL       | index | NULL          | PRIMARY | 4       | NULL |    5 |   100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
该查询是由两个单独的select进行的union,不存在子查询,所以直接把id=2的类型标记为UNION,并且第一个的select会被默认的标记为primary,之后用来union的select都会在该select之前执行。Id部分和select_type部分与union时候一致,唯独少了最后的UNION RESULT,该部分是因为要对两部分的数据排序在合并才会产生,而union all不需要排序,只需要把两部分内容合并就可以,所以不会有UNION RESULT。
 
7.2、UNION ALL依赖于外部查询
若第二个select出现在union之后,则被标记为union,并且是依赖于外部查询。
 
EXPLAIN
SELECT ID
      ,NAME
FROM   CUSTOMER
WHERE  ID IN (SELECT ID FROM EMPLOYEE UNION ALL SELECT ID FROM DEPARTMENT);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type        | table      | partitions | type   | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-------------+
|  1 | PRIMARY            | CUSTOMER   | NULL       | ALL    | NULL          | NULL    | NULL    | NULL |    1 |   100.00 | Using where |
|  2 | DEPENDENT SUBQUERY | EMPLOYEE   | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index |
|  3 | DEPENDENT UNION    | DEPARTMENT | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | func |    1 |   100.00 | Using index |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
还是和UNION一样, 只不过少了UNION RESULT部分。
 
 7.3、UNION ALL在from子句
若union all包含在from子句的子查询中,外层select将被标记为deriver(deriver表必须有别名),如果没有别名将会报错。
 
不写别名,会报错:
 
EXPLAIN
SELECT *
FROM   (SELECT ID
              ,NAME
        FROM   EMPLOYEE
        UNION
        SELECT ID
              ,DEPTNAME
        FROM   DEPARTMENT);
ERROR 1248 (42000): Every derived table must have its own alias
正确写法:
 
EXPLAIN
SELECT *
FROM   (SELECT ID
              ,NAME
        FROM   EMPLOYEE
        UNION
        SELECT ID
              ,DEPTNAME
        FROM   DEPARTMENT) T;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | PRIMARY      | <derived2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   13 |   100.00 | NULL            |
|  2 | DERIVED      | EMPLOYEE   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |   100.00 | NULL            |
|  3 | UNION        | DEPARTMENT | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    5 |   100.00 | NULL            |
| NULL | UNION RESULT | <union2,3> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.00 sec)
和union一样,不过少了UNION RESULT部分。
 
上述就是小编为大家分享的MySQL执行计划中的各个参数及含义指的是什么了,如果刚好有类似的疑惑,不妨参照上述分析进行理解。

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